One-Step Synthesis of Hierarchical Cantaloupe-like AlOOH

CRYSTAL GROWTH & DESIGN

One-Step Synthesis of Hierarchical Cantaloupe-like AlOOH Superstructures via a Hydrothermal Route Yongli Feng, Wencong Lu,* Liangmiao Zhang, Xinhua Bao, Baohua Yue, Yong lv, and Xingfu Shang

2008 VOL. 8, NO. 4 1426–1429

Department of Chemistry, Shanghai UniVersity, Shanghai 200444, People’s Republic of China ReceiVed August 14, 2007; ReVised Manuscript ReceiVed January 10, 2008

ABSTRACT: Hierarchical cantaloupe-like and hollow microspherical AlOOH superstructures were successfully synthesized on a large scale via a one-step hydrothermal route. The as-obtained superstructures were characterized by several techniques, such as X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and nitrogen adsorption/ desorption measurement. The as-obtained superstructures, consisting of closely packed nanorods in an ordered fashion, have an average horizontal axis of ca. 2.5 µm and a longitudinal axis of ca. 1.5 µm. The as-obtained cantaloupe-like AlOOH superstructures have Brunauer–Emmett–Teller (BET) surface area of about 55.5 m2/g. The possible formation mechanism of the cantaloupe-like AlOOH superstructures is proposed and discussed. Introduction Recently, one-dimensional nanostructures have attracted considerable interest due to their importance in fundamental research and potential wide-ranging applications.1–3 Their unique properties are determined by their morphology, size, and spatial arrangement and the degree of order among the individual building blocks as well. Many efforts have been focused on the intergration of one-dimensional nanoscale building blocks into two- and three-dimensional ordered superstructures or complex functional architectures (such as flower-like Cu2O, ellipsoidal ZnO, hollow microspherical In2O3, sea urchin shaped MnO2, etc.).4–7 These novel hierarchical architectures opened up possibilities for advanced nanodevices.8,9 However, systematically manipulating the morphology and architecture of inorganic materials remains a challenge to material design. Aluminum oxide hydroxide and alumina are widely used in industries as catalyst supports, adsorbents, ceramics, and abrasives due to their unique properties. AlOOH is also used as building units and directing template in the preparation of core/shell materials.10 The conventional chemical method for synthesis of Al2O3 is to dehydrate AlOOH, but the morphology of boehmite stays the same.11 The hydrothermal synthesis of fibrillar boehmite was originally studied by Bugosh12 and further developed by many research groups.13,14 Various synthetic methods of hierarchical architectures have been developed, such as vapor-phase processes and solution-based self-assembly routes,15,16 but searching for a cost-effective and easy to scale up process still remains to be the main focus. Up to now, various morphologies of boehmite have been synthesized, such as nanospheres,17 nanofibers,18,19 nanorods,20 nanotubes,21,22 nanobelts,23 bunches of aligned boehmite nanowires,24 and flowerlike 3D nanoarchitectures.25 However, to the best of our knowledge, few have reported synthesis of three-dimensional cantaloupe-like and hollow microspherical AlOOH superstructures via a one-step hydrothermal route. Herein, we report a facile hydrothermal route to fabricate the superstructures with the assistance of trisodium citrate dihydrate. Our study has indicated that trisodium citrate * To whom correspondence should be addressed. Telephone: +86-2166132663. Fax: +86-21-66134080. E-mail: [email protected].

dihydrate was essential in the formation of the cantaloupe-like superstructures. Experimental Section All chemical reagents were analytical grade and purchased from Shanghai Chemical Reagent Company (P.R. China) without further purification. In a typical synthesis, 2 mmol of Al(NO3)3 · 9H2O was dissolved in 40 mL of deionized water with vigorous stirring. Trisodium citrate dihydrate (C6H5Na3O7 · 2H2O, 0.5 mmol) was added to the solution. The mixture was further stirred vigorously for 30 min before it was sealed in a Teflon-lined stainless-steel autoclave (50 mL capacity) and heated at 200 °C for 24 h. Thereafter the autoclave was allowed to cool naturally to room temperature. The white product was centrifuged and washed several times with deionized water followed by ethanol. Finally, the product was dried at 60 °C for 12 h. The XRD patterns were recorded on a Japan Rigaku D/Max-RB X-ray diffractometer with Cu KR radiation (λ ) 1.541 78 Å). The morphologies of the samples were studied by field emission scanning electron microscopy (FE-SEM, JEOL JSM-6700F) and field emission transmission electron microscopy (TEM, JEOL JEM-2010F). Fourier transform infrared (FTIR) spectra were obtained on an AVATAR370 spectrometer. The nitrogen adsorption and desorption isotherms at 77 K were measured with a Micrometrics ASAP 3000 analyzer. Before measurement, the samples were degassed in vacuo at 200 °C for at least 6 h.

Results and Discussion The phase and purity of the products were examined by X-ray diffraction (XRD). Figure 1a shows the XRD pattern of the asprepared products. All the diffraction peaks can be readily indexed to the orthorhombic AlOOH (JCPDS Card No. 211307), no peaks from other phases can be observed, indicating the high purity of the products. The most intense peak is the (020) peak according to the standard pattern. However, the maximum intensity of the as-prepared product is observed for the (120) peak. This phenomenon may be caused by preferential growth along the (120) plane and closely packed structure in an ordered fashion. More characteristics of AlOOH are also observed in its FTIR spectrum (Figure 1b). The intensive bands at 3291 and 3093 cm-1 belong to the υas(Al)O-H and υs(Al)O-H stretching vibrations. The two weak bands at 2091 and 1970 cm-1 are the combination bands. The band at 1069 cm-1 and the shoulder at 1156 cm-1 are assigned to the δs Al-O-H and δas Al-O-H modes of boehmite, respectively.

10.1021/cg7007683 CCC: $40.75  2008 American Chemical Society Published on Web 02/28/2008

Hierarchical Cantaloupe-like AlOOH Superstructures

Crystal Growth & Design, Vol. 8, No. 4, 2008 1427

Figure 1. (a) Typical XRD pattern and (b) FTIR spectrum of the asprepared products. Figure 3. TEM and SEM images of samples synthesized at 200 °C for 24 h with different amounts of C6H5Na3O7 · 2H2O: (a) 0 mmol, (b) 1/32 mmol, (c) 1/8 mmol, (d) 1.5 mmol, (e) 4 mmol. The inset in panel d shows the corresponding SEM image.

Figure 2. (a) Low-magnification SEM image of large-scale cantaloupelike AlOOH superstructures and (b, c) enlarged SEM images of individual developed and underdeveloped cantaloupe-like microstructures.

The three bands at 746, 630, and 484 cm-1 represent the vibration mode of AlO6. The shoulder at 1640 cm-1 is the feature of the bending mode of absorbed water. These absorption bands agree precisely with the ones previously reported in the literature.26 Figure 2a shows a typical SEM image of the self-assembled AlOOH products. AlOOH cantaloupe-like patterns with the average horizontal axis of ca. 2.5 µm and longitudinal axis of ca. 1.5 µm are obtained on a large scale. The magnified SEM image shown in Figure 2b displays an individual AlOOH microparticle consisting of aligned nanorods with diameters of ca. 60 nm. As shown in Figue 2c, the interior of the individual underdeveloped AlOOH has side-by-side nanorods in an ordered

fashion. Consequently, we conclude that the microstructures are formed from the attachment of nanorods. The representative SEM and TEM images of the products prepared at different concentrations of trisodium citrate dihydrate are shown in Figure 3. Figure 3a shows the irregular nanorods obtained in the absence of trisodium citrate dihydrate. When 1 /32 mmol of trisodium citrate dihydrate was added, the as-synthesized products were composed of bunches of side-byside nanorods (see Figure 3b). As the amount of trisodium citrate dihydrate reaches 1/8 mmol, most of the products have underdeveloped cantaloupe-like structures (see SEM image shown in Figure 3c). The nanorods of the surface were also arrayed in an ordered fashion and closely packed together. With an increase in the dosage of trisodium citrate to 1/2 mmol, the as-synthesized product was monodispersed developed cantaloupe-like structures, as shown in Figure 2a. When the amount of trisodium citrate dihydrate was further increased to 1.5 mmol, the morphology of the product changed greatly and turned into hollow microspheres with diameter of ca. 2 µm (Figure 3d). The center portion of the microstructure was lighter than the edge, further confirming the hollow interior. The hollow structure was further confirmed by SEM (See the inset of Figure 3d). After trisodium citrate dihydrate amount reached 4 mmol, the product changed into a solid structure (Figure 3e). To investigate the formation process of the cantaloupe-like superstructures, we have carried out analogous experiments at different reaction durations. Figure 4 shows the typical TEM images of the sample prepared at different reaction times of 4, 6, 12, and 36 h, with other constant conditions. When the reaction time was less than 2 h, no precipitate was obtained. Loose and underdeveloped cantaloupe-like structures were formed as the result of self-assembly after the reaction time

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Figure 5. N2 absorption and desorption isotherms and pore-size distributions (inset) for the cantaloupe-like superstructures.

Figure 4. TEM images of the products prepared at different reaction stages and temperatures: (a) 4 h, 200 °C; (b) 6 h, 200 °C; (c) 12 h, 200 °C; (d) 36 h, 200 °C; (e) 24 h, 180 °C; (f) 24 h, 220 °C.

extended to 4 h (Figure 4a). The cantaloupe-like structure appeared after 6 h reaction time (Figure 4b). The morphology did not change much even when the time was further prolonged to 12 and 24 h, but the once closely packed structure became loose. The packed structure was destroyed (Figure 4d) after 36 h reaction time. This demonstrated that the cantaloupe-like structure was metastable. Normally temperature has a big impact on the crystal forms of the products. No precipitation was obtained at 160 °C. After the temperature heated to 180 °C, the underdeveloped cantaloupe-like structure appeared (Figure 4e). The products were mainly irregular nanorods at 220 °C (Figure 4f), because the strong thermal disturbance caused the interactions between the nanorods to be too weak for self-assembly. Citrate is an important biological ligand for metal ions and can form strong complexes with Ca2+, Zn2+, and Ag+ ions, etc.27 It can also serve as shape controller and stabilizer in the synthesis of Ni(OH)2,28 calcite,29 doughnut-shaped ZnO microparticles,30 etc. Self-assembly is believed to be an effective strategy to form hierarchical superstructures. Self-assembly of nanocrystals is driven by van der Waals forces and hydrogen bonding among the certain organic molecules on the surface of particles.31 It is very clear from the laboratory results that trisodium citrate dihydrate has played a crucial role in the formation of the self-assembled cantaloupe-like and hollow microspherical superstructures. Without trisodium citrate dihydrate, Al(NO3)3 · 9H2O reacted with deionized water and produced boehmite phase at higher temperature and pressure. Therefore, Al(NO3)3 · 9H2O hydrolyzed slowly to form tiny boehmite nanoparticles at the early stage because of the absence of the alkali. Citrate molecules adsorbed selectively on the surface of boehmite nanoparticles, allowing the boehmite crystallites to grow along one direction to form nanorods. Hydrogen bonds were formed between the carboxyl of trisodium citrate dihydrate and the hydroxyl ions in adjacent planes of AlO6 octahedra of boehmite.32 This led the nanorods to self-

organize into cantaloupe-like assemblies. It is believed that the integrating force in the superstructure is the strong hydrogen bonds between the contacting lateral surfaces among the nanorods. When a small quantity of trisodium citrate dihydrate was added, only a few hydrogen bonds were formed. As a result, some nanorods aligned side-by-side and formed bundles of the nanorods. Increasing the trisodium citrate dihydrate amount resulted in more hydrogen bonds. Hence, the interactions between the nanorods grew strong enough for self-assembly into the developed superstructures. When the concentration was further increased, the binding on the surface of boehmite nanoparticles was too large and it inhibited the growth along one direction to some extent. Consequently, shorter nanorods were formed and self-aggregated into hollow microspheres because of the hydrogen bonds. Determining the exact nature of the growth mechanism will require further theoretical and experimental work. Nitrogen adsorption/desorption measurement was conducted to characterize the Brunauer–Emmett–Teller (BET) surface area and internal pore structure. The recorded adsorption and desorption isotherms for the cantaloupe-like superstructures show a significant hysteresis (Figure 5). The BET specific surface area of the sample is calculated from N2 isotherms about 55.5 m2/g. Barrett–Joyner–Halenda (BJH) calculations for the pore-size distribution, derived from desorption data, present a bimodal distribution centered at ∼4 and ∼23 nm. The smaller pores presumably arise from the spaces within a nanorod. The larger pores are possibly attributed to the internanorod spaces. The results display that the obtained cantaloupe-like superstructures have porous properties. Conclusions The novel hierarchical cantaloupe-like and hollow microspherical superstructures have been successfully synthesized via a one-step hydrothermal route with the assistance of trisodium citrate dihydrate. The study suggested that the concentration of trisodium citrate dihydrate was an important factor in the formation of AlOOH superstructures. Cantaloupe-like superstructures with aligned nanorods closely packed in an ordered fashion were obtained with 1/2 mmol trisodium citrate dihydrate. The synthesis procedure presented here is simple and free of pollution, making it feasible for large-scale production. The asobtained product has a unique hierarchical cantaloupe-like superstructure and is a promising candidate in many applications, including catalysts, ceramics, abrasives, and optical nanodevices.

Hierarchical Cantaloupe-like AlOOH Superstructures

Acknowledgment. This work was supported by the National Science Foundation of China (Grant No. 20503015) and Shanghai Leading Academic Discipline Project (Project Number J50101), the Opening Subject Fund on Nano-Science of State Key Laboratory for Powder Metallurgy in Central South University. The authors thank Mrs. Lihua Chen of ZOTOS International INC for her valuable discussions and revision of the English.

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